Morphology, ligament strength, and energy absorption of nanoporous copper via vapor phase dealloying

Abstract

Nanoporous coppers (NPCs) with ligament size down to 800 nm and relative densities ranging from 15.7% to 47.5% were synthesized by vapor phase dealloying (VPD) Cu₃₃Zn₆₇, Cu₁₆Zn_84, and Cu₆Zn₉₄ precursor alloys. The ligament size of NPCs could be adjusted by applying different VPD temperatures under high vacuum, while the change in the mechanical properties of NPCs were monitored by compression tests. Although NPCs possessed a random ligament structure, the energy absorption capability of NPCs was up to 183.3 MJ/m³, which is an order higher than that of other Cu foam structures fabricated by different methods such as 3D printing, electron beam melting, electro-deposition, chemical dealloying and friction powder compaction. High apparent relative density and ligament connectivity could effectively transmit the compression energy in the form of stress waves, and therefore had higher densification strain and better energy absorption capacity. With the use of corrected Gibson and Ashby scaling equation, the ligament strength of NPC was up to 3456 MPa, which was ten times higher than that of bulk Cu and comparable to that of other ordered or periodic copper foams.

Introduction

Three-dimensional open-cell metal foams have been widely used for their high specific surface area and electrical conductivity, such as battery electrodes [1], electrochemical catalysts [2,3], actuators [4], and biosensors [5,6]. However, these applications require a certain level of mechanical strength to maintain their structural integrity and stability. It has been reported that the mechanical properties of metal foams, such as yield strengths, Young's modulus, and energy absorption ability could be adjusted by the relative density, ligament size and connectivity [7,8]. The Gibson-and-Ashby model is commonly used to predict the effective yield strength of foam metals from the relative density and the yield strength of the solid metal [9]. Furthermore, when ligament size is in nano/microscale, metal foams show higher strength than the predictions made by Gibson-and-Ashby equations [9,10]. Based on the Gibson-and-Ashby model, Jin et al. [11] further created a corrected scaling equation through the ligament connectivity between foam structures, and then the ligament strength could be calculated precisely.

Regarding the studies on mechanical properties of metal foam structures mentioned above, most of them were synthesized using chemical dealloying [12,13] or electrochemical dealloying [14,15] methods. Binary or trinary alloy precursors were selectively dealloyed into nanoporous metal foams with the use of different chemical activities of the elements. However, the chemical dealloying method can only be applied to the synthesis of nanoporous metal foams with relatively noble elements such as Au, Ag, Pt, etc. Lu et al. [16] recently developed a vapor phase dealloying (VPD) method to fabricate porous Co, while Han et al. [17] reported that porous Ge synthesized using the VPD method could be a promising anode material for Li-ion batteries. The VPD is a dealloying method that takes advantage of the different vapor pressure between two elements in precursor alloys. Under a high vacuum environment, once heating to dealloying temperature, elements in the precursor with higher vapor pressure will first vaporize. The remained element, which has relatively low vapor pressure, will rearrange to the nanoporous structure. Due to the vapor pressure difference nature, the selective removal method by VPD can transform most elements including non-metallic elements regardless of their electro conductivity into a porous structure. Moreover, vapor phase dealloying does not need additional electrolytes such as strong acid (HCl, HNO₃), base (NaOH, KOH), and other liquid-metal solvents that may harm the environment and bring extra costs to the disposal of waste solution. Another advantage of vapor phase dealloying is the convenience of recovering the sublimed element during the dealloying process. The sublimed element will deposit on the quartz tube wall outside the heating furnace and can be collected easily for other applications. Vapor phase dealloying is an eco-friendly and sustainable method to fabricate porous metals.

Recently, Pinna et al. and Shi et al. reported the analysis of mechanical properties of VPD samples [18,19]. However, several mechanical properties such as energy absorption ability, energy absorption efficiency and ligament strength have not yet been discussed. In order to apply further functional applications to VPD samples, the mechanical properties of VPD samples play an important part in material information. In this study, the Cu–Zn alloy system was chosen to fabricate nanoporous copper (NPC) by VPD method. The mechanical properties of NPC fabricated by VPD were evaluated by compression tests. Further analysis on morphology, energy absorption capacity, and ligament strength will be discussed and compared to those of ordered metal foams.